The invention relates to a process for the separation of hydrocarbons from a gaseous feedstream containing light and heavy hydrocarbons and optionally containing components boiling lower than methane. The gaseous stream is introduced to the process under elevated pressure, cooled, partially condensed, and separated into a liquid and a gaseous fraction. The liquid fraction is fractionated by rectification into a product stream containing essentially higher-boiling components and a residual gas stream containing predominantly lower-boiling components. The gaseous fraction separated after the partial condensation is introduced into a scrubbing column wherein higher-boiling hydrocarbons are scrubbed out of the gaseous fraction using residual gas obtained during the rectification as the scrubbing medium, after the partial condensation of this residual gas. The liquid fraction obtained in the bottom of the scrubbing column is fed to rectification.
Such processes serve, above all, for the removal of ethane and propane from gaseous hydrocarbon mixtures, such as natural gas or refinery waste gases. Also, these processes are suitable for the separation of analogous, unsaturated hydrocarbons, such as ethylene and propylene. Refinery waste gases contain hydrocarbons of this type, and consequently their processing has become of interest due to rising market prices for C.sub.3 /C.sub.4 hydrocarbon mixtures.
U.S. Pat. No. 4,707,171 discloses a process of the kind discussed above, wherein C.sub.2+ or C.sub.3+ hydrocarbons are separated from a gaseous mixture. A crude gas stream is partially condensed by countercurrent heat exchange with process streams which are to be heated. The partially condensed crude gas stream is separated in a separator into a liquid and a gaseous fraction. The liquid fraction consisting essentially of higher-boiling hydrocarbon components, C.sub.2+ or C.sub.3+, is fed to a rectification column wherein lower-boiling components are removed therefrom. During this rectification step, a residual gas stream is obtained at the head of the rectification column. The residual gas stream, after its partial condensation, is introduced into a scrubbing column wherein higher-boiling components are scrubbed out of the gaseous fraction discharged from the separator. The bottom fraction thus obtained in the scrubbing column is likewise introduced into the rectification column.
The scrubbing step serves to increase the yield of the process since this step makes it possible to remove from the gaseous fraction of the separator, as well as from the residual gas of the rectifying column, C.sub.2+ or C.sub.3+ components which otherwise are unobtainable.
The above-described method has the disadvantage that the required process temperatures must be provided by means of a refrigeration facility, optionally a refrigeration cascade. For this purpose, a refrigeration-producing expansion of at least part of a residual gas stream from the scrubbing step is performed.
If it is intended to subject the residual gas stream(s) obtained to further processing, high pressures must be maintained. In such a case, refrigeration is produced by circulating refrigerant media in closed cycles. However, a disadvantage of this version of the process is that it is relatively expensive.